The present embodiments relate to single photon emission computed tomography (SPECT). SPECT imaging uses a radioisotope or radiotracer to determine physiological function within a patient. A spatial distribution of the radioisotope is imaged for qualitative SPECT based on counts of emissions. For quantitative SPECT, the activity concentration or specific the uptake of the radiotracer by tissues in the body is measured. The activity concentration (i.e., the concentration of the radiotracer from different locations) is reconstructed from detected emissions. Given various modeling and unknowns in SPECT, general use of quantitative SPECT has been limited. For example, quantitative SPECT is only realized in industry for Tc-99m, which has a single emission photopeak (i.e., energy peak).
A wide range of isotopes (e.g., Lu-177, In-111, Sm-153) used in nuclear medicine emit photons with multiple photopeaks. Using a multiple photopeak radioisotope in qualitative SPECT, each photopeak is reconstructed separately, providing a separate reconstructed volume for each photopeak. The reconstructed voxels are then summed to improve signal-to-noise ratio. However, summing the output results of the reconstruction may not be useful in quantitative SPECT. Summing the output activity concentration from different photopeaks does not provide an accurate activity concentration for quantification.